The present invention relates to a control system for an internal combustion engine which is capable of detecting a spark plug fouling in the engine and controlling an operating condition of the engine on the basis of the result of detection. The present invention further relates to a control system of the above described kind for use in a direct fuel injection engine. The present invention further relates to a method carried out by the control systems.
In an internal combustion engine, an air-fuel mixture is ignited by a spark of a spark plug provided to a cylinder. As shown in FIG. 4, a usual spark plug 17 includes a metal shell lid, an insulator 17c held inside the metal shell 17d and having an end portion protruding from the same, a center electrode 17a insulated by the insulator 17c from the metal shell 17d and having an end portion protruding from the insulator 17c, and a ground electrode 17b having an end portion attached to the metal shell 17d and the other end portion disposed opposite to the end portion of the center electrode 17a so as to provide a gap g therebetween. Such a spark plug 17 is constructed so that the insulation resistance between the center electrode 17a and the ground electrode 17b (i.e., the insulation resistance of the portion schematically represented by a voltmeter V in FIG. 4) is sufficiently large.
In an internal combustion engine, there can occur such a case in which when a rich mixture is introduced successively into a cylinder, the mixture is not combusted completely due to a factor such as incomplete atomization of fuel, and so-called carbon fouling (i.e., deposition of carbon or black soot on the surface of insulator 17c) is caused. When the amount of carbon adhered to the surface of the insulator 17c becomes large, that is, when the progress of carbon fouling becomes noticeable, the insulation resistance between the electrodes 17a and 17b of the spark plug 17 becomes smaller, thus possibly causing, when a high voltage for ignition is applied from an ignition coil (not shown) to the spark plug 17, a leakage current to flow through the deposition of carbon C so that a spark is not produced at the spark gap to cause a misfire.
Further, it is known a direct fuel injection internal combustion engine having a fuel injector whose injection nozzle is disposed inside a cylinder. The fuel injector injects fuel directly into the cylinder so as to form a rich air-fuel mixture adjacent a spark gap of a spark plug and a lean mixture around the rich mixture, i.e., so as to form a stratified mixture. The mixture is combusted so as to perform a so-called stratified combustion. Since the direct injection engine enables an ignition of a mixture which is considerably lean in an average air-fuel ratio of its entirety, it has an advantage of having a good fuel consumption. However, in the direct fuel injection engine, a rich mixture is introduced to a place adjacent the spark gap. The mixture has such a characteristic that it becomes harder to be atomized sufficiently as it becomes richer. Thus, the direct injection engine has a problem in that the fuel in a liquid state tends to be adhered to the surface of the insulator and not to be combusted completely but form carbon adhered onto the surface of the insulator, i.e., tends to cause carbon fouling of the spark plug.
Thus, it has been proposed a spark plug fouling detecting method as is disclosed in Japanese Patent Provisional Publication Nos. 11-13620 and 11-336649. The method utilizes a technique of detecting ion in terms of ion current, which ion is generated when an air-fuel mixture is ignited by a spark plug and combusted. A leakage current due to spark plug fouling is superimposed on an ion current. Thus, the behavior of current detected by an ion current detecting circuit at the time of generation of ion current (more specifically, the behavior of current after the focusing of ion current) varies depending upon a variation of leakage current. The leakage current varies depending upon the progress of spark plug fouling. The method disclosed in the above described publications is adapted to detect the progress of spark plug fouling by monitoring the behavior of the current detected by the ion current detecting circuit.
In the meantime, as shown in FIG. 4, in case the progress in adherence of carbon (black soot) C to the surface of the insulator 17c is at a stage prior to causing a short circuit between the electrodes 17a and 17b of the spark plug 17, a sufficient insulator resistance is kept between the electrodes 17a and 17b though a spark plug fouling has been caused. However, when a high voltage for ignition is applied from an ignition coil to the spark plug 17, there may occur such a case in which a spark is not produced across the spark gap g but a current flows through the carbon C adhered to the surface of the insulator 17c to cause the high voltage to jump across a gap between an end portion of the carbon layer C and the inner wall surface of the metal shell 17d to create a spark which is so-called xe2x80x9cleak spark to inner shell borexe2x80x9d. Although the mixture can be ignited if located adjacent a flame kernel produced by the leak spark, such a leak spark is more difficult to be exposed to the mixture as compared with a spark at the spark gap g, thus resulting in a tendency that the combustion efficiency attained by the leak spark is lower as compared with that attained by the spark at the spark gap g.
While the method disclosed in the above described publications is adapted to detect the progress or growth of spark plug fouling, detection of the progress is made on the basis of leakage current. Generally, leakage current is caused when the spark plug fouling progresses to such an extent as to cause a short circuit and the insulation resistance between the electrodes is lowered. Thus, the method can not detect spark plug fouling until the spark plug fouling progresses to such an extend as to cause a short circuit between the electrodes of the spark plug and is therefore in a condition of causing misfires in a high probability. However, the method cannot detect spark plug fouling in a stage prior to causing a short circuit between the electrodes, i.e., spark plug fouling in a condition of causing a leak spark to inner shell bore.
For this reason, if the above described method is used for detecting the spark plug fouling and controlling the operating condition of the engine so as to inhibit the progress of the spark plug fouling, it is made to start inhibiting of the progress of spark plug fouling after the spark plug fouling has been progressed to be capable of causing a misfire in a considerably high probability. However, if it is made to start inhibiting of the spark plug fouling after the spark plug fouling has been progressed to such an extent as to cause a misfire in a considerably high probability, occurrence of a misfire due to a spark plug fouling during operation of an internal combustion engine can not be prevented or inhibited sufficiently, thus causing a possibility of lowering the performance efficiency of the engine and incurring emission of unburned gases for badly affecting the environment.
It is accordingly an object of the present invention to provide a control system for an internal combustion engine which can detect a spark plug fouling at a stage prior to causing a misfire and inhibit the progress or further growth of the spark plug fouling for thereby preventing the performance efficiency of the engine from being lowered or deteriorated by the spark plug fouling.
It is a further object of the present invention to provide a control system of the foregoing character, which is used for a direct fuel injection type engine.
It is a still further object of the present invention to provide a method carried out by the control system of the foregoing character.
To achieve the above objects, there is provided according to an aspect of the present invention a control system for an internal combustion engine comprising detecting means for detecting a discharge current flowing between electrodes of a spark plug when a high voltage for ignition is applied to the spark plug, judging means for judging whether or not the spark plug is fouled on the basis of the discharge current, and inhibiting means for inhibiting the progress of spark plug fouling when the spark plug is judged fouled.
According to a further aspect of the present invention, there is provided a control system for a direct fuel injection internal combustion engine comprising detecting means for detecting a discharge current flowing between electrodes of a spark plug when a high voltage for ignition is applied to the spark plug, judgement means for judging whether or not the spark plug is fouled on the basis of the discharge current, and inhibiting means for varying at least a fuel injection timing at which fuel is injected into a cylinder and thereby inhibiting the progress of fouling of the spark plug when the spark plug is judged fouled.
According to a further aspect of the present invention, there is provided a method of controlling an internal combustion engine comprising detecting a discharge current flowing between electrodes of a spark plug when a high voltage for ignition is applied to the spark plug, judging whether or not the spark plug is fouled on the basis of the discharge current, and inhibiting the progress of fouling of the spark plug when the spark plug is Judged fouled.